The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
What Is Morbidly Obese BMI?
Morbidly obese is a clinical term used to describe severe obesity, though it has been largely replaced by "Class III obesity" in modern NHS guidance. A BMI of 40 or above is classified as severely obese in the UK. This category carries significant health risks, but there is support available through NHS weight management services and prescription medication options. Understanding your BMI is the first step towards healthier decisions.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
BMI Categories and Morbid Obesity Ranges
Morbidly obese BMI falls into the severely obese category (Class III obesity), which the NHS defines as a BMI of 40 or above. Using the standard BMI formula (weight in kg divided by height in metres squared), a severely elevated reading indicates a significantly increased risk of weight-related health conditions.
NHS BMI categories in the UK are:
- Underweight: below 18.5
- Healthy weight: 18.5–24.9
- Overweight: 25–29.9
- Obese Class I: 30–34.9
- Obese Class II: 35–39.9
- Severely obese (Class III): 40 and above
For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, the NHS recommends lower BMI thresholds, with Class III obesity starting at a BMI of 37.5. Our free BMI calculator can help you determine your personal BMI and healthy BMI range in seconds.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Health Risks and Why BMI Limitations Matter
A severely obese BMI significantly raises the risk of type 2 diabetes, heart disease, high blood pressure, stroke, and certain cancers. However, BMI limitations are important to understand: BMI does not distinguish between muscle and fat, nor does it account for where body fat is stored (waist circumference is also crucial). A GP or healthcare professional can provide a complete health assessment beyond BMI alone.
If you fall into this category, NHS specialist weight management services (tier 3 or 4) are available in your area. These services may include dietary support, behaviour change programmes, and prescription weight loss medication options. You can use our weight converter to see your weight in different units, and our GLP-1 weight loss calculator may help you understand potential outcomes if medication is considered.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
What's the difference between morbidly obese and Class III obesity?
"Morbidly obese" is older medical terminology. The NHS and modern healthcare now use "Class III obesity" or "severely obese" to describe a BMI of 40 or above. The terms refer to the same thing—severe obesity with significant health risks—but "Class III" is the current clinical standard in the UK.
Is a BMI of 40 always unhealthy?
A BMI of 40 or above carries substantially increased health risks for most people. However, BMI is not the complete picture. Muscle mass, where fat is stored, fitness level, and overall health markers all matter. Your GP can assess your individual health context and recommend personalised support.
Can prescription weight loss medication help with severe obesity?
Yes. GLP-1 medications like Wegovy and Mounjaro, and dual GIP/GLP-1 medications, are prescription-only treatments available in the UK for eligible adults with severe obesity. These are typically accessed through NHS specialist weight management services or private prescribing. Clinical trials show they can support significant weight loss when combined with lifestyle changes.
How do I access NHS help for severe obesity?
Speak to your GP, who can refer you to a tier 3 or tier 4 specialist weight management service in your area. These services offer multidisciplinary support including dietitians, behaviour change specialists, and access to prescription medication if appropriate. Waiting times vary by region, but services are available across the UK.
What's the healthy BMI range, and how much weight loss would help?
The healthy BMI range is 18.5–24.9 (lower thresholds apply for some ethnic groups). Even a 5–10% reduction in body weight can significantly improve health outcomes—lower blood pressure, better blood sugar control, and reduced disease risk. Use our BMI calculator and progress tracker to monitor your journey.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.